Systemic lupus erythematosus (SLE) is a severe autoimmune disease that affects 5 million people worldwide. Treatment of SLB patients with corticosteroids, antimalarial, or anti-inflammatory drugs have limited efficacy. FDA-approved Benlysta, a human monoclonal antibody against B-lymphocyte stimulator, decreased disease severity in SLE patients, but African American patients did not respond to treatment. Furthermore, Benlysta caused significant side effects such as severe infections, nausea, diarrhea, and fever. Thus, there is an urgent need to develop new therapeutic strategies for lupus. Accumulating evidence supports the involvement of Toll-like receptors (TLRs) in SLE, and targeting TLRs is a promising strategy. Several TLRs (TLR2, TLR4, TLR7-9) are involved in SLE, indicating that targeting one TLR would leave signaling pathways initiated by other TLRs unaffected. This redundancy dictates the need for a more global targeting of common TLR signaling pathways for therapeutic intervention. Since IRAK4 is a critical kinase that initiates signaling by all TLRs implicated in SLE, we hypothesize that increased IRAK4 activity is the critical determinant of lupus and that attenuation of IRAK4 activity will mitigate lupus-promoting TLR pathways, providing protection against SLE, while preserving IRAK4 adapter functions that contribute to antimicrobial resistance. The hypothesis will be tested in the following Specific Aims to: 1. Define IRAK4 expression and activity during progression of SLE in lupus-prone mice; and 2. Engineer and test peptide inhibitors of IRAK4 for their ability to block SLE in lupus-prone mice. We expect to define if altered IRAK4 expression and activity underlies development of SLE in mice, mechanistically define the role of IRAK4 kinase activity by crossing IRAK4 kinase-inactive mice with lupus-prone FcR2b-/- Yaa mice, and determine the utility of new IRAK4 decoy peptide antagonists for inhibiting lupus in mice. This project is expected to provide proof-of-principle results to advance our understanding of IRAK4 signaling in SLE, and to generate the conceptual framework for a future RO1 grant to systematically analyze the role of IRAK4 expression and activity in SLE. It will facilitate computer-assisted design of small molecular components to inhibit IRAK4, and will pave the way for future translational research in SLE patients. These advances would be of key importance for basic immunology of SLE, and for improving public health of lupus patients in the U.S.